The present invention relates to police radar detectors, and more particularly, to sensitive wide band radar detectors that alert drivers to the presence of X, K, and/or wide Ka band police radar signals without responding to interfering signals such as are generated by the local oscillator (xe2x80x9cLOxe2x80x9d) of other radar detectors.
An electronic assembly for detecting the presence of police radar signals from a police radar unit is generally known, and will be referred to herein as a radar detector. In use, the radar detector is mounted in a vehicle and provides an audible and/or visual indication of the presence of a police radar signal.
Signals emitted by a police radar unit may travel a substantial distance from that unit. As is well understood, the police radar signal must travel to the vehicle under surveillance and then be returned altered by a Doppler shift representing speed of the vehicle. Microwave police radar signals lose strength as they travel over the distance between the police radar unit and the vehicle under surveillance. The greater that distance, the weaker the return police radar signal, such that at some distance and beyond, the police radar signal is too weak to return to the police radar unit and be evaluated for speed of the vehicle (detection range).
It is desirable that the radar detector capture the police radar signal while it is so weak as to be beyond the detection range of the police radar unit. However, as with the police radar unit, the greater the distance between the radar detector and the source of the police radar signal, the weaker the police radar signal. At some distance from the police radar unit, the police radar signal may be so weak that the radar detector is unable to distinguish the police radar signal from noise, meaning that a police radar signal will not be captured until the vehicle moves closer to the police radar unit. The maximum distance at which the police radar signal can still be detected and an alert given to the driver may be referred to as the capture range of the radar detector. It is clearly advantageous to design the radar detector to be sensitive and fast enough to detect police radar signals and give an alert to the driver with sufficient time to react before the vehicle is within the detection range of the police radar unit.
In order to detect very weak signals, it is necessary for the circuitry to also deal with the noise that is inherently present at low energy levels. Thus, increasing the capture range by lowering the threshold at which a signal may be considered also requires circuitry to handle the concomitant noise, otherwise the noise can be a source of unwanted and detrimental false alarms. The problems are further complicated with some of the frequencies employed for police radar signals. In particular, at higher frequencies, the police radar signal drops off or becomes weaker over ever shorter distances. Consequently, the capture range of the radar detector can become quite short, meaning that the time available to react before coming into the police radar units"" detection range grows smaller, and in some cases can be non-existent unless steps are taken to maximize the detector""s capture range.
Additionally, some police radar units are of the xe2x80x9cinstant-onxe2x80x9d type meaning that they may be used in a manner to intermittently emit only short bursts of police radar signals. These instant-on police radar units tend to be higher frequency as well. Where the bursts are only given infrequently, the first burst may be given when the radar detector is too far away to detect that burst, i.e., at that distance, the signal from the police radar unit is outside the capture range of the radar detector because it is below the sensitivity threshold of the radar detector. The second burst may come after the vehicle is within the detection range of the police radar unit. Under such circumstances, the driver will have had no advance warning that the vehicle is under surveillance. Accordingly, it is desirable to extend the capture range of the radar detector so as to enhance possible early detection of such instant-on police radar signals as well.
The circuitry and techniques utilized to detect police radar signals have become quite sophisticated, and in turn, so have the police radar units. Radar detectors must be able to quickly detect very weak signals, separate them from the noise, determine whether the signal is a valid police radar signal, and if so, give an alert to the driver. All of these functions require some amount of processing time which necessarily affects the detection range of the radar detector and might thus allow the vehicle to move towards the police radar unit until at last an alert is given.
During the 1980""s, police radar detectors typically covered only two microwave radio frequency (RF) bands, the so-called X band and K band. Radar detectors designed to deal with those two bands generally provided sufficient detection range for most situations. The recent addition of Ka band, and especially wide Ka band, police radar has complicated matters, as will be discussed.
In general, the X band is often defined to cover the frequency range of 8.00 GHz to 12.00 GHz, but more typically defined as the International Telecommunications Union (ITU) assigned band of 8.50 to 10.68 GHz. The Federal Communications Commission (FCC) of the United States allocated a portion of the X band of 10.50 to 10.55 GHz for police radar signals. Similarly, the K band is often defined to cover the frequency range of 18.00 GHz to 27.00 GHz, but more typically defined as the ITU assigned band of 23.00 GHz to 24.20 GHz. The FCC allocated a portion of the K band of 24.10 GHz to 24.20 GHz for police radar signals. As used hereinafter, the terms xe2x80x9cX bandxe2x80x9d and xe2x80x9cK bandxe2x80x9d will generally be meant to refer to the portions of the spectrum allocated to police radar signals in those bands as above described.
U.S. Pat. No. 4,313,216 (xe2x80x9cthe ""216 patentxe2x80x9d), the disclosure of which is hereby incorporated herein by reference in its entirety, sets forth an example of circuitry and techniques to detect whether a received signal is in the X band or K band, and is thus a possible police radar signal. The ""216 Patent discloses a superheterodyne receiver with a first swept local oscillator (xe2x80x9cLOxe2x80x9d) having a fundamental frequency or first harmonic, centered at 11.5583 GHz, and in a frequency range adjacent to the police radar X band (i.e., within the broadest definition of the X band, but just outside the defined police radar X band). The second harmonic of the LO is centered at 23.1166 GHz, and is similarly in a frequency range adjacent to the police radar K band (i.e., within the broader definition of the K band, but just outside the defined police radar K band). Due to the adjacency of the LO first and second harmonics to the X and K bands, respectively, when the LO signal is mixed with signals in either of those bands, there will be produced intermediate frequency (IF) signals in the same frequency range, such as centered around 1.02 GHz. The 1.02 GHz IF signals are mixed with a second LO signal, such as a fixed frequency at about 1.03 GHz, to produce 10 MHZ IF signals, which may then be dealt with by lower frequency IF circuitry, such as bandpass filters, FM discriminators and/or quadrature detectors. Due to the adjacency of the LO frequency harmonics to the X and K bands, the IF circuitry will produce pairs of closely spaced S-curves. Each associated pair of S-curves has a time positioning relative to the beginning of the sweep and a time spacing therebetween which correlates the pair to a signal in the X band or the K band, and thus allows for identification of the band of the received signal, as well as the approximate frequency thereof in that band.
Some radar detectors leak some of the RF energy generated by their LO""s. That energy could create signals that would appear to another radar detector as though they were police radar signals in the X band and/or the K band. Elimination of such nuisance signals has been accomplished by taking advantage of certain characteristics of LO signals as shown, for example, in U.S. Pat. Nos. 4,581,769; 4,750,215 and 4,862,175, the disclosures of each of which are hereby incorporated herein by reference in their entireties. Further enhancements aimed at improving the reliability and detection range of the detector include the addition of digital signal processing (xe2x80x9cDSPxe2x80x9d) such as that disclosed in U.S. Pat. No. 4,954,828, the disclosure of which is also incorporated herein by reference in its entirety.
In addition to the X band and the K band, a narrow portion of the Ka band became available for police radar use. Generally, the Ka band includes 27.00-40.00 GHz, while the ITU assigns the Ka band as frequencies between 33.4-36.0 GHz. For police radar purposes, the narrow portion of 34.2-34.4 GHz in the Ka band was first made available for police radar use. That portion of the Ka band will be referred to hereinafter as the narrow Ka band. The LO used in the detectors shown in the ""216 and ""828 patents has a third harmonic centered at 34.6749 GHz which, similar to the first and second harmonics, is in a range of frequency adjacent to the police radar band of interest, this time being the narrow Ka band (i.e., the third harmonic of the LO signal is within the broader defined Ka band, but just outside the defined police radar narrow Ka band). Consequently, the third harmonic of the existing first LO was found to be useful in also detecting narrow Ka band police radar signals using generally the same techniques as employed for X and K band police radar signals as described above.
However, the FCC has expanded the available Ka band spectrum available for police radar by defining a wide Ka band to include frequencies between about 33.4-36.0 GHz, thus including not only the narrow Ka band, but higher frequency portions of the Ka band as well. As used herein, the term wide Ka band is thus a reference to the expanded Ka spectrum available for police radar.
Availability of the wide Ka band for police radar signals created significant challenges to generally known police radar detectors. The expanded Ka band is significantly wider than the 50 to 200 MHZ range of the X, K and/or narrow Ka bands. Thus, the typical LO sweep was no longer wide enough to cover the entire wide Ka band, necessitating changes in generally known superheterodyne receivers. One particular change was to slightly change the center frequency of the LO, and vary its sweep range. In one sweep, the LO was set to sweep across a range that would include both the X and K bands, such that the first and second harmonics of the LO would produce IF signals centered around 1.02 GHz. These LO signals would be mixed with a second LO to produce 10 MHZ IF signals which could then produce associated pairs of S-curves in the IF circuitry as before, whereby to produce information correlated to the X and K bands. In another sweep, the LO sweep range was expanded such that, at the third harmonic, signals throughout the wide Ka band would mix therewith to produce 10 MHZ IF signals. As these IF signals were already at 10 MHZ, the second LO mix could be bypassed, and the S-curve pairs generated directly. The variable range LO approach is shown, for example, in U.S. Pat. No. 5,305,007 (xe2x80x9cthe ""007 patentxe2x80x9d), the disclosure of which is hereby incorporated by reference herein in its entirety.
The wide Ka band additionally posed a unique nuisance suppression challenge not previously encountered with X, K and narrow Ka band radar signals. In particular, while the first and second harmonics of the LO are still outside the police radar X band and K band, the third harmonic falls squarely within the wide Ka band allocated for use by police radar units. When other radar detectors leak LO signals, the third harmonic signals may thus appear as valid wide Ka band signals. As a consequence, it became necessary to develop circuitry and techniques by which to determine if the signal received in the wide Ka band is real (i.e., a valid police radar signal) or false (e.g., a nuisance signal such as an LO signal from another detector), otherwise drivers would likely be given many false alerts in response to nearby radar detectors. The ""007 patent also proposed a solution to the problem of false wide Ka band signals as well.
In particular, a nuisance signal in the wide Ka band caused by a leaky LO, for example, would, in addition to the third harmonic that causes the nuisance, also have one or both of a first and second harmonic that would be just outside, but in a range of frequency adjacent to, the X band or K band of interest, respectively. By contrast, a valid police radar signal in the wide Ka band would have no such related harmonic. Using that distinction, it was determined that the range of the X and K band sweep of the LO could be made large enough so as to cover not only the X and K bands, but to also cover the adjacent range of frequencies likely to include the harmonic(s) of the interfering LO. As a result, associated S-curve pairs would also be produced for these so-called xe2x80x9cinterfering Xxe2x80x9d or xe2x80x9cinterfering Kxe2x80x9d signals.
Any associated S-curve pair generated during the Ka band, especially those having a frequency correlation to that portion of the wide Ka band including the third harmonic of possible interfering LO signals, may be examined against the results from the X/K band sweep. If the wide Ka band associated S-curve pair is found to have a harmonic relationship with an associated S-curve pair correlated to either or both of an interfering X or an interfering K signal from the X/K band sweep, then the wide Ka band signal may be rejected, whereas if no such harmonic relationship is found, then the signal received during the wide Ka band sweep may be considered to be from a police radar unit and an alert given. The ""007 patent thus discloses and claims rejection of wide band Ka signals having a harmonic relationship with signals in either or both of the X band and/or the K band.
While the nuisance signal rejection accomplished with the techniques disclosed and claimed in the ""007 patent is quite advantageous, further improvements are desired, including advances to increase the capture range of the radar detector. U.S. Pat. No. 5,852,417 (xe2x80x9cthe ""417 patentxe2x80x9d) relates to a radar detector that is said to have increased sensitivity, and hence increased capture range, through the use of Low Noise Amplifiers (LNA""s) operating in the X, K and/or Ka bands. While such (LNA""s) are believed to enhance sensitivity, the radar detector circuitry of the ""417 patent presents other undesirable performance characteristics. In particular, the radar detector of the ""417 patent utilizes an LO having harmonics that are not adjacent to either the X or the K bands, and is instead well outside of even the broadest definitions of these bands. As a result, while there may be improved sensitivity, the radar detector of the ""417 patent lacks the ability to either directly determine band from associated S-curve pairs for each detected signal, or to reject wide Ka band signals due to the presence of either one of a harmonically related X band or K band signal. With respect to band detection, the LO frequency is so far removed from the band of interest that IF signals produced in the radar detector of the ""417 patent do not produce associated S-curve pairs which can be used to identify the band of the received signal. Instead, additional signals must be injected by the radar detector into the IF signals to determine the band of the received signal.
With respect to rejection of wide Ka band signals, the LO fundamental frequency of the ""417 patent radar detector is selected such that the range of frequency adjacent to the X band, where the first harmonic of most interfering LO signals would reside, is not captured. Thus, the first harmonic of interfering LO signals cannot be detected by the circuitry of the ""417 patent. Instead, the LO frequency is selected such that only interfering K signals may be detected, meaning that a wide Ka band signal is rejected only if the harmonically related K band interfering LO signal is also detected. Thus, while the radar detector of the ""417 patent utilizes the concepts of the ""007 patent to ignore a wide Ka band signal when there is a harmonically related signal in the K band, that solution is not entirely satisfactory. By way of example, the system of the ""417 patent can be fooled by nuisance signals leaking from radar detectors that have K band image filters. Those devices, while still emitting the first harmonic, will generally suppress the second harmonic of the LO. The radar detector of the ""417 patent can not detect the first harmonic of the LO and so will give an alert in response to detection of the third harmonic from such radar detectors as if a valid police radar signal in the wide Ka band had been received.
Therefore, a significant need exists for improving sensitivity of a police radar detector without sacrificing the advantages of an LO that is at a frequency adjacent to either one or both of the lower frequency bands of interest (e.g., the X and K bands), including the suppression of false wide Ka band signals afforded thereby.
The present invention provides a police radar detector that takes advantage of the sensitivity improvements offered by LNA""s, but without sacrificing the advantages of operating with an LO having harmonics at frequencies adjacent to at least the X or K bands. To this end, and in accordance with the principles of the present invention, an adjacent frequency LO is swept three separate times, one for each of the X, K, and wide Ka bands, and a pair of IF circuits produce for each such sweep, respective first and second pairs of S-curve or IF signals. The respective first pairs of IF signals produced from the two X and K band sweeps are correlated to signals in those bands, whereas the respective second pair of IF signals produced from those two sweeps are correlated to interfering X and interfering K signals, respectively, such as would be produced by the first harmonic or second harmonic of an interfering LO. When the wide Ka band is swept, the first pairs of IF signals are correlated to signals from a portion of the wide Ka band that would not be expected to include LO third harmonics, while the second pairs of IF signals would be correlated to signals in a second portion of the wide Ka band which could include both valid police radar signals and/or interfering third harmonic LO signals.
The use of separate sweeps and dual IF circuits provides several advantages. In accordance with one aspect of the present invention, there are provided separate X band and K/Ka band LNA""s which are selectively energized depending on the band to be swept. The separate sweeps thus allow for the use of such tuned LNA""s, so as not to require the use of a single amplifier, for example, to handle the broad spectrum of X, K and wide Ka bands. Such a single amplifier would likely be very large and expensive and inherently noisy over some portions of those bands, thereby adversely affecting sensitivity or capture range. In accordance with a second aspect of the invention, dual IF circuits are both responsive to signals detected during each sweep. The use of separate sweeps for each band and dual IF circuits allows for immediate and automatic identification of the band of the received signal based on which sweep is undertaken, with the further advantage that wide Ka band signals can be promptly determined to be valid or false by comparison of the harmonic relationship or lack thereof between the second pairs of IF signals from the wide Ka band sweep with either or both of the second pairs of IF signals from the X and/or K band sweeps. Thus, the radar detector of the present invention provides the advantages of the wide Ka band spurious signal rejection of ""007 patent, while taking advantage of this improved sensitivity provided by the ""417 patent, but without the drawbacks associated therewith.
By virtue of the foregoing, there is thus provided a police radar detector which has improved sensitivity without sacrificing the advantages of an LO that is at a frequency adjacent to at least the lower bands of interest, including the suppression of false wide Ka band signals afforded thereby.
These and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.